First off, if you do not have a monitor like a Scangauge II, you will not be able to see this. Also, the odds that it will happen to you are VERY LOW, but it is possible. It happened to me the first time at 115K miles,
For those of you with a monitor - DO NOT shut your car off during this "Pre-Regen" period. It will cause a DPF Full message and send you to the dealer. My theory is that it's heating up the exhaust but it's not yet hot enough to start burning off the soot, and the extra fuel will be mistaken by the sensors as extra soot. I had my grams of soot go from 22-35 immediately once and this is why. 32 Grams is the threshold to force a manual regen which at this time can only be done by a dealership. Also, if you drive too far with the "DPF Full" and "Reduced Power" message, it is possible to ruin the DPF.
Here's a video. The "Pre-Regen" is before a regen is indicated by a 1 on the gauge, but when the MAP is below 14, or the boost is negative.